1. Field of the Invention
The present invention is generally related to composite membranes, and more particularly to composite polytetrafluoroethylene membranes.
2. Description of the Prior Art
Polytetrafluoroethylene (PTFE) is a highly crystalline polymer containing two types of chemical bonds, C—C and C—F bonds. These two types of chemical bonds are strong, given the bond energy of the C—C bond about 290-335 kJ/mole and that of the C—F bond about 482 kJ/mole. Thus, PTFE has a very stable chemical property and an excellent electrical insulation property and has also low friction coefficients. In addition, PTFE can be used at 250° C. for a long period of time and is unaffected by solvent or chemical substance, when being operated below its melting point. PTFE has many applications, such as for electrical insulation, corrosion-resistance and abrasion resistance.
Due to difficulty in dissolving PTFE in common solvents, traditionally PTFE porous films have been made by extruding PTFE powders into paste rods, calendering the rods to form films, and stretching the films to make them porous. Such porous films are also called expanded polytetrafluoroethylene (ePTFE) membranes. However, the application of ePTFE membranes to membrane separation is limited to microfiltration, because the pores generated by stretching are usually larger than 0.1 μm and not uniform. To extend the application of PTFE membranes for separating particles smaller than 0.1 μm, such as ultrafiltration, new membrane fabrication process are still needed to manufacture PTFE membranes with smaller pores and more uniform size distribution.